The use of ceramic particles for reinforcing, strengthening, and toughening metallic, polymeric or ceramic matrix materials has grown increasingly important in recent years. Such particles are typically shaped as whiskers, fibers, platelets or the like. For example, silicon carbide whiskers are used to reinforce alumina or silicon nitride matrices for cutting tools. The beta-crystalline form of silicon carbide has been found to be especially useful for these purposes, since it has excellent mechanical properties, good thermal conductivity, and good oxidation resistance.
The use of beta-silicon carbide whiskers has encountered drawbacks, however, most notably in the methods of making the whiskers. For example, silicon carbide whiskers have been made from rice hulls (chaff) by heating the hulls to temperatures high enough to react silicon contained in the hull structure with carbon in the hulls. Methods of this sort typically include coking the rice hulls before their conversion to silicon carbide. Unfortunately, such methods are usually of low efficiency; excess carbon must typically be burned off from the product, and the average silicon carbide whisker content of the burned-off product is only 15-20 percent. While differential wetting before burn-off can improve the free-flow characteristics, deagglomeration processes can be time consuming and can require significant capital investment in separation equipment. Making sure that the whiskers are free-flowing is important because agglomerated whiskers won't evenly disperse into a matrix.
A variety of efforts have been made to improve the efficiency of methods for producing silicon carbide particles. For example, JP 62-036100 and JP 61-022000 disclose methods for preparing silicon carbide whiskers by heating mixtures including silicon nitride, carbon and a catalyst such as iron, nickel or cobalt metal or oxide. JP 61-291498 and JP 61-257000 disclose methods for manufacturing silicon carbide whiskers from a gaseous silane in the presence of a metallic catalyst (such as titanium) in the form of a metallic powder, or in the form of a gaseous metal or metal compound, respectively. JP 63-103899 and JP 60-260496 disclose methods for making silicon carbide whiskers from carbonaceous and silicic powders or gases, respectively, each entailing the decomposition of a transition metal compound gas (for example, ferrocene) as a catalyst. While all of these methods have generally been successful for their intended purposes, they are typically slow, inefficient and expensive, and often yield silicon carbide particles of mixed alpha- and beta-phases and inhomogeneous composition.
Efforts have also been made to form silicon carbide whiskers directly in situ in a silicon nitride matrix. For example, JP 60-064970 and JP 60-064969 disclose methods of forming silicon carbide whiskers in a silicon nitride article, but only from elemental silicon in the article, and not from the silicon nitride making up the article. In JP 60-064970, the elemental silicon is an unreacted residual, left over from the partial nitriding of a shaped silicon body, while in JP 60-064969, the elemental silicon is impregnated into a sintered or calcined porous silicon nitride.
While each of these two methods avoids the free flow and dispersion problems previously encountered when separate silicon carbide whiskers were added to a silicon nitride matrix, they each inconveniently require a separate treatment step of the matrix material before the silicon carbide whiskers can be formed. This increases the cost and complexity of obtaining the desired end product, and may make the product less homogeneous or more subject to contamination.
It would be desirable to form silicon carbide whiskers within a silicon nitride matrix or from silicon nitride contained in some other matrix. It would also be desirable to obviate the inefficiencies and inhomogeneity encountered in prior methods for making the whiskers themselves.
Therefore, it would be advantageous to provide a high-yielding method for the rapid and economic formation of free-flowing beta-silicon carbide whiskers, especially whisker particles of superior smoothness and uniformity of diameter and size, with a greater percentage of whiskers being in the beta-crystalline form, and less non-whisker particles or whiskers in alpha-crystalline form, than present in previous products.
In addition, it would be especially advantageous to provide a method of directly forming beta-silicon carbide whiskers in situ within a silicon nitride matrix from the silicon nitride itself which makes up the matrix.